This invention relates to a pressure relief valve for rail tank cars and similar or analogous containers with high and variable internal pressures. More specifically this invention relates to a pressure relief valve that can be preset at two quantitatively different pressures so that volatile contents inside a rail tank car or other transportable or stationary container can be released into the environment under two different sets of physical conditions. The purpose of a pressure relief valve is to allow the contents of the tank to escape such that the pressure within a tank cannot rise to the point where the tank shell will rupture. Most specifically this invention relates to a pressure relieve valve with two groups of spring assemblies, and wherein (i) one group of spring assemblies contains mechanical attaching components (ii) that melt at a different temperature than that of the mechanical attaching components of the remaining spring assemblies and (iii) thereby allowing for two different preset pressures.
U.S. Pat. No. 5,855,225 (Williams) discloses (i) four constant force spring assemblies (ii) aligned in a square configuration along a spring plate and (iii) mechanically attached to this same spring plate. The spring assemblies are preset to a specific pressure (tension/force) (i) at which the internal pressure from liquid or gas within the rail tank car or other similar transportable or stationary container to which the pressure relief valve is attached (ii) will overcome the spring assembly force upon a sealing disk and (iii) dislodge the sealing disk vertically from the opening within the valve flange.
The first problem with the Williams valve is that it cannot                (i) be preset to dislodge the sealing disk at a lower predetermined pressure,        (ii) in addition to the standard operating pressure and temperature resulting from the total number of operatively connected constant force spring assemblies.        
Instead, the devices that mechanically attach the spring assemblies to a spring plate, and thereby provide opposing force that retains a sealing disk upon the vent aperture of the valve, are made of metal that does not melt at the appropriate temperatures. Consequently, William's mechanical attaching devices only provide venting of the tank or container content at a higher pre-set pressure. They also do not provide adequate venting in conditions where the rail tank car or other transportable or stationary container is engulfed in a fire or when the rail tank car shell or other container shell ruptures.
The invention described herein solves this first problem with fusible mechanical attaching devices to operatively attach force generating spring assemblies to a spring plate. These fusible mechanical attaching devices include, but not exclusively (ii) bolts, screws, or other mechanical connecting devices (iii) that are made of materials such as, although not exclusively, rigid plastics, rigid solid nylon and other nylon blends. The invention operatively positions these fusible devices along the elevated walls of a spring plate in a pre-determined pattern with conventional bolts made of metal.
Consequently, in the current invention most constant force spring assemblies attach to the spring plate elevated walls by mechanical devices with a significantly lower melting temperature than those spring assemblies attaching with conventional metal attaching devices. When the temperature within or outside the rail tank car or other transportable or stationary container reaches a predetermined value, the fusible attaching devices melt. This melting mechanically disconnects corresponding constant force spring assemblies from the spring plate. The remaining two opposing constant force spring assemblies continue to operative connect the spring plate with metal attaching devices above this specific melting point.
More specifically, the centrally positioned constant force spring assembly in each opposing set of three spring assemblies attaches to the spring plate with metal attaching devices. The remaining attaching devices of the spring assemblies to the spring plate elevated walls are made of a rigid material with a melting temperature below the temperature that would cause the rail tank car or other transportable or stationary container to over-pressurize and explode in a high energy event. The two attaching devices that oppose each other and remain attached to corresponding spring assemblies and spring plat elevated walls are preferably made of steel or stainless steel. With only two operatively connected spring assemblies providing opposing force to the pressure from the heated rail tank car or similar container contents, the sealing disk dislodges at a lower pressure setting and the rail tank car or other transportable or stationary container content escapes prior to a potential explosion.
Another problem with the Williams valve occurs because the vertical rigid brackets of the William's pressure relief valve limits the number of constant force spring assemblies that can be placed within the valve enclosure. In contrast, the improved bracket design described herein allows for linear adjacent placement of additional spring assemblies along a spring drum and enclosed spring drum bolt; this additional length for operative placement of additional spring assemblies. These additional spring assemblies allow the dual set pressure valve to increase its potential opposing force to the contents of a rail tank car or similar transportable or stationary container.
More specifically, the linear distance of four inches between the Williams brackets limits the number of two inch wide spring assemblies that can be placed adjacent to his disclosed two spring assemblies. The current invention described herein solves this problem with design of a new vertical spring bracket. This new spring bracket design and structure accommodates up to three two inch wide spring assemblies adjacent to each other on opposing elevated walls of the spring plate of the valve. Furthermore, longer spring drums and spring drum bolts of the current invention assist in accommodating a total of ten spring assemblies (ii) while the Williams valves can only physically and structurally accommodate a maximum of eight spring assemblies.
In the current invention there are preferably ten spring assemblies, each of which is two inches in width. Each spring assembly is mounted upon two adjacent spring brackets with a partially stepped structure. In contrast, with the Williams valve design each spring bracket is straight vertically and linear in a uniform rectangular shape. This rectangular configuration allows only two spring assemblies between two adjoining brackets. However, in the current invention each spring bracket has a panel that is one-inch offset (i.e., the right bracket panel has a one inch offset to the right and the left bracket panel has a one-inch offset to the left) approximately one and one-half inches above the spring bracket end that attaches to the circular valve flange. This offset provides sufficient space for a third two inch wide spring assembly. Thus, each of two opposing sides of the valve can accommodate an additional two inch wide spring assembly with sufficient clearance for a wrench to attach the valve to a rail tank car or similar container mounting flange.